Golf balls lately have not been limited only to one-piece golf balls and two-piece golf balls; a variety of balls having multilayer structures that include a multilayer cover or multiple covers have been proposed. Active efforts are being made to develop golf balls in which the formation of such a multilayer cover or multiple covers dramatically increases flight performance and also provides numerous advantages overall relating to, for instance, the feel of the ball when played, controllability on approach shots, and durability. In balls having multiple covers or a multi-layer cover, a smaller thickness per cover layer increases the degree of freedom in ball design, enabling golf balls of outstanding flight performance and other properties to be obtained. Such a golf ball having a thin cover is manufactured by supporting the core within a mold cavity and injection molding the cover into what remains of the cavity. However, the molten resin often does not flow well throughout the cavity, resulting in molding defects detrimental to the physical properties of the ball. It is particularly difficult to injection mold a very thin cover having a thickness of 1.0 mm or less, and so obtaining high-quality balls with satisfactory ball characteristics such as sphericity has been a challenge. To this end, various innovations, such as modifying the mold configuration by changing the size and increasing the number of mold parts, e.g., injection gates and vent pins, have been proposed. Unfortunately, this approach leads to higher mold fabrication costs and increases the labor associated with the manufacturing process.
U.S. Pat. No. 3,640,028 and JP-A 2000-14832 disclose grinding methods for removing molding flash from the surface of a golf ball or core and for reducing the ball or core diameter. However, these are not suitable as grinding methods for creating high-precision, ultrathin covers. That is, an excessive grinding force is unnecessary and inappropriate for deflashing. Diameter reduction does require a certain degree of grinding force, but such prior-art grinding methods are ill-suited for precisely finishing the entire spherical surface of the cover to a small thickness.
JP-A 2001-309996 describes the formation of a cover to a predetermined thickness by machining, but does not disclose a manufacturing method for obtaining balls of a high sphericity. Moreover, it is not possible to achieve a high sphericity simply by randomly grinding the surface of the cover layer. Because even a slight departure from a spherical shape may very well destabilize the flight performance and roll of a golf ball, and also decrease the rebound, worsen the feel on impact and reduce the durability, there exists a need to obtain balls of high sphericity.